SA was actually a target of propaganda war waged by Ussr in 1983. There were a lot of fake letters, redacted by KGB, which tried to demonstrate and denounce links between SA and Western Powers, in order to discredit the Western leaderships (http://www.csmonitor.com/1983/0922/092239.html/(page)/2). In case of a general war, Ussr would probably have tried to intervene in African wars, against SA. In case of protracted conflict, this could have been a useful propaganda tool ("We, the Socialist countries, are against the Evil Apartheid Regime"). In case of thermonuclear global war the destruction of SA could be less useful. But... I don't know if there was a Soviet African Strategy in case of nuclear war. But in the 40s and 50s there was, for exemple, a France African Strategy in case of nuclear war: if France is destroyed by a nuclear weapons, French people and army could regroup and reorganize in African colonies. Maybe the Soviets could have thought the same thing when they tried to create new friendly regimes and bases in Sub-Saharan Africa: a safe haven, useful for a post-war reconstruction. Destroying a South African enemy could make sense in this scenario, also in case of thermonuclear global war.